In an existing Long Term Evolution (LTE) system, when a terminal device needs to transmit uplink data, the terminal device uses an uplink scheduling request (SR) mechanism. That is, the terminal device sends an uplink scheduling request to a base station, and informs the base station that the terminal device requires an uplink resource for data transmission. After receiving the scheduling request from the terminal device, the base station allocates specific resources to the terminal device. The terminal device transmits data on the allocated resources.
Development of the Internet of Things has resulted in an increasing quantity of machine type communication (MTC) services. For an MTC service, generally, a data packet is relatively small, and a relatively low transmission delay is required. When a large quantity of MTC services exist, the foregoing scheduling request mechanism not only causes high signaling overheads but also leads to a transmission delay.
To resolve the foregoing problem, an uplink grant-free transmission technical solution has been proposed. Grant free means that, in a Public Land Mobile Network (PLMN), a terminal device does not need to use a scheduling request to request a base station to allocate a resource for data transmission. A grant free user can directly transmit a packet on a contention transmission unit (CTU) according to different characteristics, such as transmission delay or reliability requirements, of to-be-transmitted data.
However, in some scenarios, for example, when a base station (source base station) accessed currently by a terminal device carries excessively heavy load or has relatively poor channel quality, or when a terminal device enters a coverage area of a base station from a coverage area of another base station, the terminal device needs to be handed over from the source base station to another base station (target base station) to perform grant-free transmission.
Therefore, how to hand over a terminal device from a base station to another base station to perform grant-free transmission is a problem that urgently needs to be resolved.